PATCH DYNAMICS OF EELGRASS ZOSTERA-MARINA

Eelgrass has declined extensively during the last decades following eu trophication of coastal regions of western Europe and the USA. Recent efforts have been taken to reduce nutrient loading, with the hope of r estoring the former widespread populations. We studied fine-scale patc h dynamics of eelgrass Zostera marina L. in permanent plots outside th e continuous vegetation in a protected embayment in Limfjorden, Denmar k, to evaluate means of recruitment and rates of expansion and mortali ty of patches. The size distribution was dominated by small patches wh ich were formed by seedlings at high rates during spring (0.16 to 0.76 M-2). Mortality was high and only low proportions (0 to 24 %) of the studied cohorts remained as individual patches 1.5 and 2.5 yr later. P atch mortality was restricted to small patches containing < 32 shoots with a mean age of < 5 yr. The sharply declining mortality with increa sing patch age and size is presumably due to improved anchoring, mutua l physical protection and physiological integration among the shoots. The lateral expansion of established patches by centrifugally growing horizontal rhizomes averaged 16 cm yr-1 and was independent of patch s ize. With this lateral growth the possible annual areal expansion will be faster in systems with many small patches (260 % in 0.1 M2 patches ) and slower in systems with few large patches (19 % in 10 M2 patches) . Successful recovery of the eelgrass vegetation in large systems is, therefore, dependent on seed production and dispersal and subsequent s eedling establishment and patch growth.